1. Field of the Invention
This invention relates generally to nucleic acid sequences encoding proteins that are associated with abiotic stress responses and abiotic stress tolerance in plants. In particular, this invention relates to nucleic acid sequences encoding proteins that confer drought, cold, and/or salt tolerance to plants.
2. Background Art
Abiotic environmental stresses, such as drought stress, salinity stress, heat stress, and cold stress, are major limiting factors of plant growth and productivity. Crop losses and crop yield losses of major crops such as rice, maize (corn) and wheat caused by these stresses represent a significant economic and political factor and contribute to food shortages in many underdeveloped countries.
Plants are typically exposed during their life cycle to conditions of reduced environmental water content. Most plants have evolved strategies to protect themselves against these conditions of desiccation. However, if the severity and duration of the drought conditions are too great, the effects on plant development, growth and yield of most crop plants are profound. Furthermore, most of the crop plants are very susceptible to higher salt concentrations in the soil. Continuous exposure to drought and high salt causes major alterations in the plant metabolism. These great changes in metabolism ultimately lead to cell death and consequently yield losses.
Developing stress-tolerant plants is a strategy that has the potential to solve or mediate at least some of these problems. However, traditional plant breeding strategies to develop new lines of plants that exhibit resistance (tolerance) to these types of stresses are relatively slow and require specific resistant lines for crossing with the desired line. Limited germplasm resources for stress tolerance and incompatibility in crosses between distantly related plant species represent significant problems encountered in conventional breeding. Additionally, the cellular processes leading to drought, cold and salt tolerance in model, drought- and/or salt-tolerant plants are complex in nature and involve multiple mechanisms of cellular adaptation and numerous metabolic pathways. This multi-component nature of stress tolerance has not only made breeding for tolerance largely unsuccessful, but has also limited the ability to genetically engineer stress tolerance plants using biotechnological methods.
Therefore, what is needed is the identification of the genes and proteins involved in these multi-component processes leading to stress tolerance. Elucidating the function of genes expressed in stress tolerant plants will not only advance our understanding of plant adaptation and tolerance to environmental stresses, but also may provide important information for designing new strategies for crop improvement.
There are at least four different signal-transduction pathways leading to stress tolerance in the model plant Arabidopsis thaliana. These pathways are under the control of distinct transcription factors, protein kinases, protein phosphatases and other signal-transduction pathway components (Shinozaki et al. 2000 Curr. Op. Pl. Biol. 3:217–23). These proteins are prime targets for engineering stress tolerance since they could function as master switches; alterations in a single gene would lead to activation of an entire signal-transduction chain leading to stress tolerance.
Sensing of osmotic stress in bacteria as well as in plants is performed by a two-component system comprising a sensing protein and an effecting protein (Wurgler-Murphy S M and Saito S. 1997. Trends in Biochem. Sci. 22:172–6 and Shinozaki et al. 2000. Curr. Op. Pl. Biol. 3: 217–23). Mitogen-activated protein kinase-dependent signal transduction pathways are tightly involved in these processes. Another major component of these signal-transduction chains are GTP-binding proteins (G-proteins). Generally speaking, there are at least three classes of G-proteins: a) heterotrimeric (alpha, beta and gamma subunits), b) monomeric (small) proteins, and c) Dyanins. GTP-binding proteins are named as such because each must bind GTP in order to be active. The functions of GTP-binding proteins are varied as they range from directly transmitting an external signal (by being associated with a membrane-bound receptor), to participating in vesicle traffic, to importing proteins into sub-cellular compartments.
The participation of trimeric G-proteins in stress tolerance has not yet been directly demonstrated. However, since they are associated with membrane-bound receptors, they may be involved in transmitting the sensed stress signal leading to stress tolerance. Binding of the ligand to a receptor could conceivably cause the activation of the alpha subunit and thereby change the concentration of a low-molecular weight second messenger. Changes in the concentration of second messengers, like cAMP in animal systems, ultimately activate further components of the signal transduction pathway and second messengers such as inositol derivatives have been implicated in stress tolerance in plants (Ishitani et al. 1996 PI Journal 9:537–48).
Monomeric/small G-proteins are also involved in many different cellular processes and have been implicated in vesicle traffic/transport, cell cycle and protein import into organelles. Several groups have identified small G-proteins, homologous to the Rab family of small G-proteins, as being induced upon desiccation treatments in plants (Bolte et al. 2000 Plant Mol. Biol. 42:923–36; O'Mahony and Oliver 1999 Plant Mol. Biol. 39:809–21). These researchers speculate that the small G-proteins could be involved in preservation of membrane integrity or re-structuring upon relief of stress. However, they have not produced transgenic plants with increased stress tolerance by over-expression of these small G-proteins.
Another class of G-proteins is the high-molecular weight G-proteins. Dynamins are a representative member of this class. There have been several homologs of high molecular weight G-proteins identified in a number of eukaryotic systems, including yeast, human, rat and mouse and in plant systems including Arabidopsis thaliana, Nicotiana tabacum and Glycine max. Although sequence and proposed functions of the different homologs are diverse, they all appear to function in protein trafficking, likely assisting in vesicle formation. The most thoroughly characterized Dynamin to date was isolated from rat and has been shown to bind to microtubules and participate in endocytosis while being regulated by phosphorylation (Robinson et al. 1993 Nature 365:163–166.). Additionally, plant Dynamin isolated from Glycine max was found to be localized across the whole width of the newly formed cell plate during cytokinesis, suggesting a role for the protein for depositing cell plate material via exocytic vesicles (Gu &Verma 1996. EMBO J. 15:695–704). The protein, ADL1, isolated from Arabidopsis, has been shown to be localized to the thylakoid membranes in sub-organellar fractionation of chloroplasts from leaves. Transgenic plants over-expressing a mutant form of this gene show a reduced number of chloroplasts and those existing chloroplasts had a reduced number of thylakoids and thylakoid membrane proteins. These data suggest a role for ADL1 in transport of protein needed for biogenesis of thylakoid membranes (Park et al. 1998 EMBO J. 17:859–867). One other such homologue, DNM1, found in Saccharomyces cerevisiae also participates in endocytosis, acting at a novel step before fusion with the late endosome (Gammie et al. 1995. J. Cell Biol. 130:553–566). Despite this research however, the participation of Dynamins in stress tolerance has not been demonstrated.
There is a need, therefore, to identify genes expressed in stress tolerant plants that have the capacity to confer stress resistance to its host plant and to other plant species. Newly generated stress tolerant plants will have many advantages, such as increasing the range that crop plants can be cultivated by, for example, decreasing the water requirements of a plant species.